The OPNsense® project invites developers to start developing with OPNsense:
“For your own purpose or even better to join us in creating the best FreeBSD
based open source firewall available!” The development workflow & build process
has been redesigned to make it more straightforward and easy for developers to
build OPNsense.

Being able to get the sources and build it yourself is one of the key factors of
open source software. One reason that for starting the OPNsense project is that
the team believes sources and build tools should be freely available and as easy
to use as possible.